There is no easy answer to this question. Your issue goes beyond business. This is more about disappointment, betrayal and humiliation than it is about dollars and cents--or dollars and sense. It is hard enough for any man to lose his business. Believing that he is losing it to his son adds insult to injury. Attempt to sit down and have a heart to heart talk with your father. Listen more then talk. Ultimately, he may or may not be able to see the logical benefit to you taking his clients as opposed to having them go elsewhere. If he cannot give you his blessing then you will have to decide which is more important--your relationship with your parents or building your business. Remember, if you do not pursue these clients and they go elsewhere you can always try to win them over in the future, once your father has had a chance to heal from this episode.
Best of luck,